1. Field of the Invention
This invention relates generally to a surface emitting laser diode, and more particularly to a low threshold, surface emitting distributed feedback laser diode.
2. Description of the Prior Art
The double heterostructure semiconductor laser diode provides optical waveguiding and carrier confinement in an active layer and is the basis for most modern laser diodes. One of the common methods of stimulating laser light emission is by the creation of a population inversion in the semiconductor with current injection in the gain media. If the applied current injection exceeds a threshold level, electron-hole pairs are stimulated and recombine to emit light with the direction and phase of the light in the waveguide of the resonant cavity. The electrical-to-optical conversion efficiency (differential quantum efficiency) for injection pumped lasers can be as high as 70% after the onset of threshold current. Thus, injection pumped semiconductor laser diodes are extremely attractive for a wide range of uses in optical and electroptical applications.
However, the laser threshold current is a function of, among other things, temperature. The threshold current density in a cleaved cavity laser diode is given by: EQU J.sub.th =4.5.times.10.sup.3 d/g+(20d/gC)(a.sub.i +(1/L.sub.cavity)ln(1/R))
where:
g=quantum efficiency PA1 C=confinement factor PA1 R=intensity reflectance PA1 d=active layer thickness PA1 a.sub.i =intrinsic absorption coefficient PA1 L.sub.cavity =laser cavity length PA1 q=electron charge PA1 m.sub.neff =electron effective mass PA1 x=radiation angular frequency PA1 n=average refractive index of the semiconductor. PA1 L.sub.cavity =length of the resonator cavity PA1 n.sub.GaAs =refractive index of laser active region PA1 m=integer PA1 k=wavelength
Furthermore, the refractive index of the preferred semiconductive materials, such as AlGaAs or GaAs, is a function of temperature and injected current. The refractive index as a function of temperature is approximately given by: EQU dn(t)=4.times.10.sup.-4 dt
and the dependence on current (i) is: EQU dn(t)=-(i*q.sup.2 /2m.sub.neff .times..sup.2)n
where:
Accordingly, the refractive index increases with increasing temperature and decreases with increasing current.
The lasing wavelength of a cleaved cavity laser diode is directly proportional to the mechanical length of the laser cavity, which in turn is also a function of temperature. The lasing light has to satisfy the condition: EQU mk/2=n.sub.GaAs L.sub.cavity
where:
Hence, the laser output beam wavelength varies; the typical output characteristic of a laser diode according to temperature of the device consists of sections with a linear slope and discontinuous "mode hops" where the wavelength changes by one cavity mode spacing (C/2L). Changes in wavelength of the diode under current modulation also occur. Such instability in the output wavelength is quite undesirable and decreases the coherence of the laser light. Coherence is a necessary feature of the output beam in that the laser light is often made to interfere with itself, which is important in interferometers and coherent communications systems.
Normally, light incident on an aperture is diffracted with the outermost angle being inversely proportional to the size of the aperture (i.e. smaller angles cause greater divergence). However, typical active layer thicknesses are on the order of 0.2 to 0.3 micrometer in a double heterostructure laser and are as small as 100 angstroms for quantum wells. As a result, typical half angle divergences H(fwhm) are 25 to 35 degrees perpendicular to the active layer.
Because the light output of a typical laser diode comes from a non-symmetrical aperture, the output beam is undesirably non-symmetrical as well. Typically, the angular divergence perpendicular to the active layer is 2 to 5 times the divergence parallel to the active layer.
Thus, two significant factors in the construction of a laser diode are the aperture and divergence of the emitted beam. Conventional laser diodes typically emit through an aperture of 0.2.times.5 micrometers, which is the result of the use of an extremely thin layer of semiconductive material, forming an active region, which is kept thin to eliminate the possibility of emission intensity distributions with higher order modes than the TEM.sub.00 mode. The width of the aperture is either determined by a current blocking oxide stripe or a refractive index waveguide fabricated into the laser. Thin (0.2-0.3 micrometer) active layer construction is often used because such construction can decrease the lasing threshold current density.
Divergence in an edge-emitting laser diode is inversely proportional to the aperture size, and thus beam divergence is greatest in a direction perpendicular to the active layer. This causes the output from the laser to diverge at large angles, especially in edge-emitting laser diode. Typical edge-emitting laser diodes have half angle divergences of 34 degrees perpendicular to the active layer and 8 degrees parallel to the active layer. These wide divergence angles necessitate the use of a collimating lens with a high numerical aperture to refract the light into a plane wave. For comparison, it should be noted that gas (for example, HeNe) and solid-state lasers have output beams which are already collimated with divergences on the order of milliradians. Laser diodes constructed to provide such a mode of emission (surface mode), but with low divergence output, are be potentially useful in a large number of applications. Such devices have not been easily achieved, however.
The foregoing and other disadvantages of edge-emitting laser diodes therefore make them unsuitable for many applications. For example, in an area such as fiber optic communications, the foregoing disadvantages impede the use of edge emitting laser diodes, as the dispersion and absorption of glass fibers are minimized at certain wavelengths (1.3, 1.5 lm) and therefore variations in wavelength cause phase delays and pulse broadening. Laser wavelength mode hops are also associated with undesirable intensity noise in optical data storage devices such as compact optical storage disks.
When used in conjunction with an adjacent p/n junction, the wavelength of a laser diode can be tuned by changing the current in the adjacent diode. A surface emitting laser diode so constructed would allow for fiber optic communications with multiple beams of differing wavelength propagating in the same fiber. The beams could then be demultiplexed using a diffractive focusing feature which disperses the beam and focuses it onto an array of detectors, with one detector for each channel and each channel corresponding to a different wavelength.
Accordingly, a wavelength-tunable, low current threshold, low divergence surface emitting laser diode would be extremely attractive for use in in coherent optical communications, position measuring devices based on interferometers with outputs similar to linear encoders, optical systems using holographic optical elements whose properties are wavelength dependent, and for illuminating compact holographic optical disk read/write heads, holographic-based laser deflectors (hologons), and laser lenses.
Single or multiple versions of such a low threshold, low divergence surface emitting laser diode could also be formed along with other components such as GaAs MESFET's or photodiodes to form highly-useful integrated optical systems. The combination would also simplify the use of diffractive input/output couplers.